1. Field of the Invention
The present invention relates in general to polymer thick film compositions. More particularly, it relates to solventless, UV hardenable, thermally curable resistor paste compositions.
2. Description of the Related Arts
Typically, polymer thick film resistor pastes contain conductive particles dispersed in a resin formulation. The uncured resistor paste is usually screen-printed or extruded through a syringe-like apparatus in the desired circuit pattern. The resistor paste is then cured by application of heat or UV radiation.
To achieve the desired low viscosity necessary for processing, conventional polymer thick film resistor pastes contain solvents to reduce viscosity. However, the conventional solvent based pastes may shrink up to 50% on curing due to solvent evaporation. This dimensional instability creates an uneven surface in the resistor pattern and cracks in the conductive fillers. Furthermore, contacts with metal electrodes may be broken as the shrinking volume pulls the resistor pattern away from the electrodes, thus creating electrical shorts.
One difficulty in developing UV curable resistor pastes is that the conductive fillers, such as carbon black or silver particles, do not transmit UV radiation. As a result, the interior regions of the resistor pattern are shielded from UV radiation and cannot be completely cured in the UV radiation step. In the case of thermally curable resistor pastes, shapes of the uncured resistor patterns are liable to distort during transportation, or due to variations of curing temperature. Thus, it is difficult to produce resistor patterns without deformation.
Most commercially available UV curable resins are inadequate for circuit writing application. For example, UV cured acrylates have poor adhesion to metals. Also, UV cure of acrylates is oxygen inhibited, resulting in a tacky surface due to incomplete curing. Further, silver (which is the standard conductive filler material) may catalyze polymerization of many formulations at room temperature before exposure to UV radiation. Once polymerized, these acrylate formulations cannot be used to write circuit patterns.